The correct answer is option 'D' - Tropic of cancer, summer solstice. Let's understand why this is the correct answer.

Explanation:

Tropic of Cancer:
The Tropic of Cancer is an imaginary line located at 23.5 degrees north of the equator. It is the northernmost point where the sun's rays can be directly overhead during the summer solstice in the Northern Hemisphere.

Summer Solstice:
The summer solstice is the day with the longest period of daylight in the year. It occurs around June 21st in the Northern Hemisphere and December 21st in the Southern Hemisphere. During this time, the tilt of the Earth's axis is inclined towards the sun, resulting in the Northern Hemisphere experiencing summer.

Vertical Sun Rays:
When the sun's rays are "vertical," it means they are directly overhead at a particular location. This occurs when the sun is at its highest point in the sky for that location. At this point, the sun's rays are most intense and the shadows cast by objects are shortest.

Understanding the Answer:
Given that the question asks for the location and time when the sun rays are vertical, we need to identify the correct combination of the Tropic and the season.

- Option 'A' - Tropic of Capricorn, equinox: The equinox occurs around March 21st and September 21st, when day and night are of equal length. However, the sun's rays are not vertical over the Tropic of Capricorn during these times.
- Option 'B' - Tropic of Cancer, winter solstice: The winter solstice occurs around December 21st in the Northern Hemisphere. During this time, the sun's rays are not vertical over the Tropic of Cancer. They are actually vertical over the Tropic of Capricorn.
- Option 'C' - Tropic of Capricorn, summer solstice: The summer solstice occurs around June 21st in the Northern Hemisphere. While the sun's rays are vertical over some location on this day, it is not the Tropic of Capricorn. The correct answer should have the Tropic of Cancer.
- Option 'D' - Tropic of Cancer, summer solstice: The summer solstice in the Northern Hemisphere occurs around June 21st, and during this time, the sun's rays are indeed vertical over the Tropic of Cancer. This is the correct answer.

Therefore, option 'D' - Tropic of Cancer, summer solstice is the correct answer.

On 21st June, the northern hemisphere is tilted towards the sun. The rays of the sun fall directly on the tropic of cancer. As a result, these areas receive more heat. The areas near the poles receive less heat as the rays of the sun are slanting. The north pole is included towards the sun and the places beyond the Arctic circle experience continuous day light for about six months. Since a large portion of northern hemisphere is getting light from the sun, it is summer in the regions north of the equator. The longest day and the shortest night at these places occur on 21st June. This position of the earth is called Summer solstice. So, the answer is D.